I started working on disassembling the US 2.00W BIOS a couple weeks ago. It's all still in an IDA Pro database file at the moment, but I'll see if I can't chop it up and put it on GitHub or something this weekend.

This article on Kaneda's front page of SpritesMind should get people started on basic technical information about the Sega CD's operation.

Also, from the same developer is a SD card loader replacement for the Sega Saturn called Rhea. From a hardware standpoint, a Saturn is essentially a Sega Genesis CD 32x with additional, and better unified hardware, and I believe the CDC protocal in the Sega CD and Sega Saturn are similar. Might be worth to see what is similar between both CDC protocals and what SD Card loader hardware/code techniques could be adapted/retrofitted to the Sega CD.